Earth leakage also described as "ground fault" is often caused by an insulation defect causing circuit current to pass directly to ground. Differential current transformers and zero sequence current transformers are commonly used within the phases of a multi-phase electrical distribution surface to sum the current within each phase and to determine in the absence of zero current that there is unwarranted transport between one or more phases and the neutral phase which is electrically connected to ground.
An early example of a circuit breaker using a burden resistor connecting to the output of a differential transformer for earth fault detection is found in British Patent GB2109163 entitled "Earth Leakage Circuit Breaker for Electrical Installation Protection". A further example of a circuit breaker having earth fault protection is found in Russian Patent SU 1644279 entitled "Cable Network Earth Leakage Current Protection Device". This patent teaches the use of a capacitor filter circuit to eliminate so-called "nuisance tripping."
Recent attempts employing bandwidth filters to determine the occurrence of ground fault within residential circuits are found in U.S. Pat. No. 5,224,006 entitled "Electronic Circuit Breaker with Protection Against Sputtering Arc Faults and Ground Faults"; and U.S. patent entitled "Low Cost Apparatus for Detecting Arcing Faults and Circuit Breaker Incorporating Same".
When such filter circuits are used within 50 Hz electrical distribution circuits for first order low pass filters, the values of the resistors and capacitors are selected to allow the detector circuit to have a low sensitivity to harmonics, particularly the third harmonic. For 60 Hz electrical circuits, the values must be adjusted accordingly to provide the same degree of third harmonic attenuation.
It would be advantageous to determine earth leakage by use of a common filter circuit for both 50 Hz and 60 Hz with the same or higher degree of harmonic attenuation of the harmonics within the associated electrical circuit.
One purpose of the invention is to provide an earth leakage protection circuit for use with both circuit breakers and relays within both 50 Hz and 60 HZ electrical distribution circuits with the same or higher degree of harmonic attenuation of the harmonics within the associated electrical circuit, without substantial increase in circuit component cost.